This invention relates to fibers formed of ethylene-propylene copolymers and, more particularly, to such fibers and processes for their preparation.
Isotactic polypropylene is one of a number of crystalline polymers that can be characterized in terms of the stereoregularity of the polymer chain. Various stereospecific structural relationships, characterized primarily in terms of syndiotacticity and isotacticity, may be involved in the formation of stereoregular polymers for various monomers. Stereospecific propagation may be applied in the polymerization of ethylenically-unsaturated monomers, such as C3+alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinyl compounds such as vinyl aromatics, e.g. styrene or vinyl chloride, vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g, isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is probably of most significance in the production of polypropylene of isotactic or syndiotactic structure.
Isotactic polypropylene is conventionally used in the production of fibers in which the polypropylene is heated and then extruded through one or more dies to produce a fiber preform which is processed by a spinning and drawing operation to produce the desired fiber product. The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula: 
Stereoregular polymers, such as isotactic and syndiotactic polypropylene, can be characterized in terms of the Fisher projection formula. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene, as shown by Formula (2), is described as follows: 
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes which may be characterized by the following formula:
Rxe2x80x3(C5(Rxe2x80x2)4)2HfQpxe2x80x83xe2x80x83(3)
In Formula (4), (C5 (Rxe2x80x2)4) is a cyclopentadienyl or substituted cyclopentadienyl group, Rxe2x80x2 is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and Rxe2x80x3 is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1-20 carbon atoms and p is 2.
Metallocene catalysts, such as those described above, can be used either as so-called xe2x80x9cneutral metallocenesxe2x80x9d in which case an alumoxane, such as methylalumoxane, is used as a co-catalyst, or they can be employed as so-called xe2x80x9ccationic metallocenesxe2x80x9d which incorporate a stable non-coordinating anion and normally do not require the use of an alumoxane. For example, syndiospecific cationic metallocenes are disclosed in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, the metallocene cation is characterized by the cationic metallocene ligand having sterically dissimilar ring structures that are joined to a positively charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Similar relationships can be established for isospecific metallocenes.
Canadian Patent Application No. 2,178,104 to Peiffer discloses propylene polymers prepared in the presence of isospecific catalysts incorporating heavily substituted bis(indenyl) ligand structures and the use of such polymers in forming biaxially oriented polypropylene films. As described in the Canadian application, the polymers used have a very narrow molecular weight distribution, preferably less than three, and well-defined uniform melting points. In each case the ligand structures are substituted on both the cyclopentyl portion of the indenyl structure (at the 2 position), and also on the aromatic portion of the indenyl structure. The tri-substituted structures appear to be preferred, and less relatively bulky substituents are used in the case of 2-methyl, 4-phenyl substituted ligands or the 2-ethyl, 4-phenyl substituted ligands.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or as unsupported catalysts, sometimes referred to as homogeneous catalysts. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components. Traditional supported catalysts are the so-called xe2x80x9cconventionalxe2x80x9d Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride, as disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. A supported catalyst component, as disclosed in the Myer ""718 patent, includes titanium tetrachloride supported on an xe2x80x9cactivexe2x80x9d anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Myer ""718 is employed in conjunction with a co-catalyst such and an alkylaluminum compound, for example, triethylaluminum (TEAL). The Myer ""717 patent discloses a similar compound that may also incorporate an electron donor compound that may take the form of various amines, phosphenes, esters, aldehydes, and alcohols.
While metallocene catalysts are generally proposed for use as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described in the Welborn ""432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, zirconia and the like. Non-metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The Welborn ""561 patent discloses a heterogeneous catalyst that is formed by the reaction of a metallocene and an alumoxane in combination with the support material. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a xe2x80x9cconventionalxe2x80x9d supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamshoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. Nos. 5,308,811 to Suga et al and 5,444,134 to Matsumoto.
The polymers normally employed in the preparation of drawn polypropylene fibers are normally prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in the aforementioned patents to Myer et al. U.S. Pat. Nos. 4,560,734 to Fujishita and 5,318,734 to Kozulla disclose the formation of fibers by heating, extruding, melt spinning, and drawing from polypropylene produced by titanium tetrachloride-based isotactic polypropylene. Particularly, as disclosed in the patent to Kozulla, the preferred isotactic polypropylene for use in forming such fibers has a relatively broad molecular weight distribution (abbreviated MWD), as determined by the ratio of the weight average molecular weight (Mw) to the number average molecular (Mn) of about 5.5 or above. As disclosed in the Kozulla patent, the preferred molecular weight distribution, Mw/Mn, is at least 7.
A process for the production of polypropylene fibers formed from isotactic polypropylene prepared through the use of isospecific metallocene catalysts is disclosed in U.S. Pat. No. 5,908,594 to Gownder et al. As disclosed in Gownder, the polypropylene is characterized in terms of 0.5-2% of 2-1 insertions and has an isotacticity of at least 95% meso diads. This results in intermittent head-to-head insertions to provide a polymer structure that behaves somewhat in the nature of a random ethylene/propylene copolymer resulting in a variable melting point. The resulting fibers have good characteristics in terms of mechanical properties and machine operation, including machine speed.
In addition to fibers formed from stereoregular propylene homopolymers such as isotactic polypropylene, alpha olefin copolymers, or fibers made with synthetic resin fibers may also be formed from alpha-olefin copolymers. Thus, U.S. Pat. No. 4,909,975 to Sawyer et al discloses the production of fibers formed from ethylene-C3-C12 alpha olefin copolymers. In the Sawyer procedure the alpha olefin co-monomers copolymerized with the ethylene are C3-C12 alpha olefins and preferably C4-C8 alpha olefins with one octene being particularly preferred. The alpha olefin co-monomer or mixture of co-monomers, which is copolymerized with ethylene, is usually present in an amount of about 1-30 wt. %. Stated otherwise, the ethylene content of the ethylene-alpha olefin copolymer can range from about 70 to 99% in the copolymers employed in Sawyer.
In accordance with the present invention there is provided a process for the production of fibers formed from ethylene-propylene copolymers. In carrying out the invention there is provided a random copolymer of ethylene and isotactic polypropylene having a random ethylene content within the range of 2-12 mole %. The propylene content of the copolymer preferably has an isotacticity of at least 90% meso diads. The copolymer is heated to a molten state and then extruded to form a fiber preform. The fiber preform is subject to a spinning speed, preferably of at least 500 meters per minute. This is followed by drawing the spun fiber preform at a draw ratio within the range of 1:1-6:1 to produce a continuous fiber of the ethylene-propylene copolymer. Preferably the ethylene-propylene copolymer has an ethylene content within the range of 3-8% and more preferably within the range of 6-8%.